Power-driven grass mowers capable of cutting grass with a rotary circular blade or cutter are very popular not only in domestic gardens but also in agricultural fields. This type of power-driven grass mower is commonly equipped with a small engine. However, the use of an engine is noisy. Noise in the residential areas is likely to cause a nuisance which is condemned as a typical public hazard. In addition, the engine-operated mower violently vibrates, thereby making it difficult for elders, women and children to use it. Even for men, the violent vibration can be a problem when the mower is used for a long period of hours, especially in agricultural fields. In recent years, motor-driven grass mowers are employed in place of engine-driven grass mowers.
A motor-driven grass mower requires an electric power source, and the commercially available motor-driven grass mowers use a commercial electric power source, wherein the mower is connected to the power source through an electric plug. The mower must be equipped with an electric wire in order to receive a current supply from the power source, and the wires must be long enough to enable the mower to work in areas distant from the power source, and move around here and there in a vast field. However, it often happens that the wires are inadvertently cut by the rotary blade of the mower while it moves around.
In order to eliminate the wire-cut troubles occurring while the mower is in operation, a battery has been replaced by the commercial power source. Examples are disclosed in Japanese Utility Model Laid-open Application No. 1995/36611 (Reference (1)) and Utility Model Registration No. 2,525,820 (Reference (2)).
The grass mowers disclosed in References (1) and (2) are driven by motor, and the torque of the motor is transmitted to the rotary cutter. The motors of these grass mowers rotates at constant speed irrespective of a load applied to the cutter.
In general, the trouble arises in the power-driven grass mowers: in the case of engine-driven mowers, while it is cutting grass, the cut pieces are likely to be caught in the rotating cutter; as a result, the cutter are locked or stopped from rotating. In this situation, the built-in clutch is disengaged, thereby causing the engine to idle. The presence of the clutch is advantageous in keeping the machine parts from becoming damaged.
On the contrary, in the case of motor-driven grass mower, the torque of the motor is directly to the cutter with no clutch interposed in the power transmission passage. Therefore, when the cutter is locked for some reason or other, the motor is forced to stop. Nevertheless, the motor continues to receive the supply of electric current. The electricity excessively builds up in the motor under the electrical characteristics of the motor. As a result, the battery is rapidly consumed for no purpose. The sooner the battery is consumed, the shorter the working hours become.
Accordingly, the primary object of the present invention is to save electricity if the cutter is locked for some reason or other.